The performance and thrust output of a marine drive unit is highly dependent on the design characteristics of the propeller. Variations in size, pitch, rake, number of blades, etc., greatly affect the thrust output, or performance, of a marine drive unit. Therefore, it is highly desirable to be able to accurately measure the thrust output for various propellers.
It is commonly known that in a marine drive unit, the thrust output of a propeller is of equal magnitude to the force applied to the opposite end of the propeller shaft against the marine drive housing. With this in mind, the thrust output of any propeller may be measured by the force of the shaft against the housing.
Prior to the present invention, individual strain gauges, made of fine, highly resistive wires, were carefully arranged and attached with epoxy to the longitudinal length of the propeller shaft. Normally, a total of four gauges were required to receive an acceptable signal. The four gauges were divided into two sets, each placed 180.degree. apart on the shaft circumference. Within a set, one gauge was laid parallel to the axis, and the other, circumferentially. Further, each gauge was laid out sinusoidally to create four periods for additional sensitivity. However, since these strain gauges rotated with the propeller shaft, a slip ring assembly was required to transfer the signal created by the thrust of the propeller from the rotating strain gauges to stationary lead wires.
Each of the four strain gauges requires an associated slip ring, and each slip ring consists of three parts. The first part of a slip ring assembly is a signal conducting transfer material attached to the circumference of the shaft and electrically connected to the strain gauges. The second slip ring component is a signal conducting brush assembly which is stationary with respect to the rotating shaft and the rotating signal conducting transfer wire. It is preferred to construct the signal conducting brush with a high grade silver coating because of the need to have both electrical signal conduction and high friction wear. A brush material of carbon composition may also be used. The signal conducting brush acts to transfer the signal from the rotating shaft to the stationary housing. The third part of a slip ring is the output lead which has one end electrically connected to the signal conducting brush, and the other to an external signal amplifier.
There are several problems associated with the use of strain gauges and slip rings for measuring the forces applied to rotating marine drive propeller shafts. These problems include small signal output of the strain gauges with relation to high noise interference which is caused by the rotational movement of the strain gauges against the slip ring brushes and a relatively high number of electrical connections. This noise is inherent in this multi-piece configuration and severely limits the sensitivity of this device.
In order to achieve a signal that is higher than the noise created by the slip ring configuration and the amplifier, it is necessary to hollow out the shaft to create a thin walled shaft in the area of strain gauge contact to create higher amplitude signals. However, the hollowed shaft tends to bend which is undesirable in a marine drive propeller shaft and can result in false thrust readings.
A further limitation of the strain gauge and slip ring assembly is that the slip ring is subject to extreme, rapid wear and replacement costs are relatively high.
A further limitation of strain gauges and slip rings is that since the signal output of this gauge is quite low compared to the noise level developed in the slip rings, strain gauges are not effective when only a limited portion of the shaft's thrust exerting surface is available. In other words, the strain gauge and slip ring configuration's signal-to-noise ratio is highly dependent on surface area contact. The lower the contact area, the higher the noise level compared to the signal.